1. Field of the Invention
The present invention relates to emissive imager devices comprising a monolithic semiconductor arrays of multicolor laser emitters that can be used as an image sources in digital projection systems.
2. Prior Art
The advent of digital display technology is causing a phenomenal demand for digital displays. Several display technologies are poised to address this demand; including Plasma Display Panel (PDP), Liquid Crystal Display (LCD), and imager based projection displays that use micro-mirrors, a liquid crystal on silicon (LCOS) device or a high temperature poly-silicon (HTPS) device (Ref. [33]). Of particular interest to the field of this invention are projection based displays that use imager devices, such as those mentioned, as an image forming device. These types of displays are facing strong competition from PDP and LCD displays and as such are in critical need for effective means to improve their performance while significantly reducing their cost. The primary performance and cost driver in these types of displays are the imagers used, such as micro-mirrors, LCOS and HTPS devices. Being passive imagers, such devices require complex illumination optics and end up wasting a significant part of the generated light, which degrades the performance and increases the cost of the display system. The objective of this invention is to overcome the drawbacks of such imager devices by introducing an emissive imager device which comprises an array of multicolor laser emitters that can be used as an image source in digital projection systems.
FIGS. 1A and 1B are block diagram illustrations of typical projector architectures 100 used in projection display systems that use a passive imagers, such as those that use reflective imagers including micro-mirrors or LCOS imager devices (FIG. 1A) and those that use a transmissive imager, such as HTPS imager devices (FIG. 1B); respectively. In general, the projector 100 of a typical projection display system of FIG. 1A is comprised of an imager 110, illuminated by the illumination optics 120 which couples the light generated by the light source 130 onto the surface of the imager 120. The light source 130 can either be a lamp that generates white light or a semiconductor light source, such as light emitting diodes (LED) or laser diodes, that can generate Red (R), Green (G) or Blue (B) light.
In the case of the projector 100 that uses a reflective imager illustrated in FIG. 1A, when a lamp is used as a light source, a color wheel incorporating R, G and B filters is added between the illumination optics and the imager to modulate the required color. When a semiconductor light source is used in conjunction with a reflective imager, the color is modulated by turning on the semiconductor light source device having the required color, being either R, G or B.
In the case of a projector 100 that uses the transmissive imager illustrated in FIG. 1B, when a lamp is used as a light source, the illumination optics 120 includes optical means for splitting the white-light generated by the lamp into R, G and B light patches that illuminate the backsides of three HTPS imager devices and a dichroic prisms assembly is added to combine the modulated R, G and B light and couple it on the projection optics 140.
The projection optics 140 is optically coupled to the surface of the imager 110 and the drive electronics 150 is electrically coupled to the imager 110. The optical engine generates the image to be projected by modulating the intensity of the light generated by the light source 130, using imager 110, with the pixel grayscale input provided as image data to the drive electronics 150. When a reflective imager (FIG. 1A) such as micro-mirror or LCOS imager device is used, the drive electronics provides the pixel grayscale data to the imager 110 and synchronizes its operation either with the sequential order of the R, G and B segments of the color wheel, when a white light lamp is used as a light source, or with the sequential order in which the R, G or B semiconductor light source is turned on. When a transmissive imager such as the HTPS imager device is used, the drive electronics provides the pixel grayscale data to the imager 110 and synchronizes the operation of each of the R, G and B HTPS imager devices in order to modulate the desired color intensity for each pixel.
Typically the losses associated with the coupling of light onto the surface of imager 110 are significant because they include the intrinsic losses associated with the imager 110 itself, such as the device reflectivity or the transmissivity values, plus the losses associated with collecting the light from the light source 130, collimating, filtering and relaying it to the surface of the imager 110. Collectively these losses can add up to nearly 90%; meaning that almost 90% of the light generated by the light source 130 would be lost.
In addition, in the case of a reflective imager 110 such as micro-mirror or LCOS imager devices, the imager 110 being comprised of a spatial array of reflective pixels, sequentially modulates the respective colors of the light coupled onto its pixelated reflective surface by changing the reflective on/off state of each individual pixel during the time period when a specific color is illuminated. In effect, a typical prior art reflective imager can only modulate the intensity of the light coupled onto its pixelated reflective surface, a limitation which causes a great deal of inefficiency in utilizing the luminous flux generated by the light source 130, introduces artifacts on the generated image, adds complexities and cost to the overall display system and introduces yet another source of inefficiency in utilizing the light generated by the light source 130. Furthermore, both the reflective as well as the transmissive type imagers suffer from an effect known as “photonic leakage” which causes light to leak onto the off-state pixels, which significantly limits the contrast and black levels that can be achieved by these types of imagers.
As stated earlier, the objective of this invention is to overcome the drawbacks of prior art imagers by introducing an emissive imager device comprising an array of multicolor laser emitters that can be used as an image source in digital projection systems. Although semiconductor laser diodes have recently become an alternative light source 130 (Ref. [1]-[4]) for use in projectors 100 of FIG. 1A to illuminate reflective imagers 110 such as the micro-mirror imager device, the use of semiconductor laser diodes as a light source does not help in overcoming any of the drawbacks of prior art imagers discussed above. In addition numerous prior art exists that describes projection displays that uses a scanned laser light beam to generate a projection pixel (Ref. [5]-[6]).
Prior art Ref. [7] describes a laser image projector comprising a two dimensional array of individually addressable laser pixels, each being an organic vertical cavity laser pumped by an organic light emitting diode (OLED). The pixel brightness of the laser image projector described in prior art Ref. [7] would be a small fraction of that provided by the pumping light source, which, being an OLED based light source, would not likely to offer an ample amount of light, rendering the brightness generated by the laser projector of prior art Ref. [7] hardly sufficient to be of practical use in most projection display applications.
Although there exist numerous prior art references that describe laser arrays (Ref. [8]-[30]), no prior art was found that teaches the use of multicolor laser emitters as pixels in an imager device. As it will become apparent in the following detailed description, this invention relates a separately addressable array of multicolor laser pixels formed by optically and electrically separating a monolithic layered stack of laser emitting semiconductor structures. With regard to creating an optically and electrically separated (isolated) semiconductor laser emitter array, Ref. [10] teaches methods for forming a single wavelength laser semiconductor structure with isolation regions (i.e. physical barriers) between the light emitting regions formed by either removing material between the light emitting regions or by passivating the regions between the light emitters of the semiconductor structure. However, the methods described in Ref. [10] could only be used to create a one-dimensional linear array of separately addressable single wavelength laser emitters within the range of wavelength from 700 to 800 nm.
With regard to creating an array of separately addressable multicolor laser emitters, Ref [21] describes an edge emitting array of red and blue laser structures. Although Ref. [21] deals with multicolor laser structure, it is only related to a two-color one-dimensional linear array of edge emitting laser structures.
Although Ref. [22] describes a display system that uses an array of vertical cavity surface emitting laser (VCSEL) diodes, because of the inherent size of the VCSEL diodes described in Ref. [22], the approach described would tend to produce substantially large pixels size because of the inherent size of the multiple color of VCSEL diodes it uses which are arranged side-by-side in the same plane to form a pixel array, rendering it not usable as an imager device.
Given the aforementioned drawbacks of currently available imager devices, an imager that overcomes such weaknesses is certain to have a significant commercial value. It is therefore the objective of this invention to provide an emissive imager device comprising a monolithic semiconductor 2-dimensional array of multicolor laser emitters that can be used as an image source in digital projection systems. Additional objectives and advantages of this invention will become apparent from the following detailed description of a preferred embodiments thereof that proceeds with reference to the accompanying drawings.